I have a Canon 700d and a 430EXIII flash. Recently I have been experimenting with off-camera flash. I was really surprised to see e-ttl ii available even when using off-camera flash (via the pop-up flash)
Now, I understand (or at least I think I understand) what TTL does (or whatever Canon calls it) when the flash is mounted on the camera.
- Flash sends a preflash and sees what reaches the lens
- A proper exposure is calculated
- Flash adjusts its power to reach that proper exposure
- Actual flash is shot along with shutter
However if the flash is off-camera, step 3 is no longer straight-forward. I may have placed the flash on my scene in a position where it is impossible to obtain a proper exposure no matter the power of the flash.
As a quick example, let's say that I want to use the flash as a back-light for a portrait.
During the pre-flash nothing will really change on what is shown through the lens apart from some small highlights on the hair of the person.
So if the flash assumes that it needs a lot of power to obtain the proper exposure (because let's say that the face of my model is really underexposed) its batteries will be wasted as it goes to full power since 99% of its light never changes the picture.
Is my assumption correct?
Or does TTL behave differently when on-camera and off-camera?